Dual fuel compression ignition engine

ABSTRACT

A dual fuel compression ignition engine (10) comprises a gas fuel supply system (20-25) for the engine (10), and a diesel fuel supply system (12-16) for the engine, including an injection pump (11) having a governor and a control member to control the amount of diesel fuel injected into the engine by the pump (11), and means (13&#39;) to arrest movement of the control member (11a) in the direction to increase the supply of diesel fuel to the engine, at a position to supply pilot fuel at a constant, or substantially constant, rate, the position for arrest being determined in accordance with at least one operating condition of the engine, such as the engine speed. Another embodiment shows a present rail system. In both, the amount of pilot diesel fuel is adjusted by a feed-back signal indicative of the amount of pilot fuel injected.

DESCRIPTION OF INVENTION

This invention relates to a dual fuel compression ignition engine. Inthis specification the term "compression ignition engine" is intended torefer, not only to an engine operating on a constant pressure, i.e.diesel cycle, but also an engine operating on a compression ignitioncycle. In such an engine, during dual fuel operation, a gaseous fuel ismixed with air and the mixture is inducted into the engine. Thetemperature and pressure prevailing at the end of the compressionstroke, whilst they would be sufficient to ignite diesel oil, areinsufficient to ignite the gas/air mixture. Accordingly, to achieveignition, a small amount of diesel oil is injected into the engine usinga conventional injection pump and injector system. This small amount ofdiesel oil, hereinafter referred to as pilot fuel, ignites andconsequently ignites the gas/air mixture so that combustion thereoftakes place.

It is desirable to inject the minimum amount of pilot fuel in order tominimise the proportion of diesel fuel used since diesel fuel is moreexpensive than gas.

Practical experience shows that a suitable ratio at full throttleoperation for a typical automotive diesel which it is desired to convertto operate on gas is around 10% diesel oil and 90% gas in terms of theenergy provided by the respective fuels.

The injection of the required amount of pilot fuel is achieved bypositioning a control member, conventionally referred to as the rack, inan injection pump of the type which pumps discrete volumes or "slugs" offuel to the injector system, to the position appropriate for the amountof fuel. It has been found that if the control member is maintained in afixed position to give the desired amount of pilot fuel at full throttleoperation, the amount of pilot fuel injected per stroke of the engineincreases significantly, and sometimes very substantially, with enginespeed.

An object of the invention is, therefore, to provide a dual fuelcompression ignition engine wherein the above mentioned problem isovercome or is reduced and, more generally, more accurately to controlthe amount of pilot diesel fuel injected.

According to one aspect of the present invention we provide a dual fuelcompression ignition engine comprising a gas fuel supply system for theengine, a diesel fuel supply system for the engine, including aninjection pump having a governor and a control means to control theamount of diesel fuel injected into the engine by the pump, and means toarrest movement of a control member, in the direction to increase thesupply of diesel fuel to the engine, at a position to supply pilot fuelat a constant, or substantially constant rate, said position beingdetermined in accordance with at least one operating condition of theengine.

The position of the injection pump control member can thus bedetermined, for example from rig testing, to be in its optimum positionto give good ignition with the minimum amount of pilot fuel.

Said at least one operating condition of the engine may comprise theengine speed.

The position at which movement of the control member of the pump isarrested can be determined, not only on the basis of engine speed, butalso by other operating conditions such as the position of the speedcontrol (i.e. throttle) of the engine, alone or in combination with theengine speed.

If desired a feed-back signal may be used in determining the position atwhich movement of the control member is arrested.

The feed-back signal may be indicative of the amount of pilot fuelinjected into the engine.

The feed-back signal may be dependent upon the temperature of the pilotfuel, or the actual power developed by the engine.

If desired other means may be provided to generate a feed-back signal,which signal may be used to check that the amount of pilot diesel fuelactually being injected into the engine corresponds with that requiredfor the position of the control member in accordance with the operatingcondition.

According to another aspect of the present invention we provide a dualfuel compression ignition engine having a gas fuel supply system for theengine, a diesel fuel supply system for the engine, including aninjection pump and a control means to control the amount of pilot dieselfuel injected into the engine and first means to provide a first inputto the control means to cause the control means to supply pilot fuel ata predetermined rate in accordance with at least one operating parameterof the engine and second means to provide a second input to said controlmeans to modify the supply of pilot fuel signalled by the first means inaccordance with at least one further parameter of the engine.

Said at least one operating parameter of the engine may comprise thespeed of the engine.

Said at least one further parameter of the engine may comprise afeed-back signal.

The feed-back signal may be indicative of the amount of pilot dieselfuel actually being injected into the engine and the control means maybe adapted to adjust the supply of the pilot fuel to correspond to thatsignalled by the first input.

The injector pump may be of the type which pumps discrete volumes offuel to the injector system.

In one embodiment, the feedback signal is derived from a strain gaugewhich determines the pressure of the diesel fuel in a feed line from theinjection pump, means being provided to correlate the pressure with theamount of fuel injected into the engine from the injection pump.

For example the strain gauge may be provided on a membrane on which thediesel fuel acts.

In another embodiment instead of a strain gauge the or one or more ofthe injectors through which fuel is injected into the engine, may beprovided with an accelerometer to sense the opening and/or closing ofthe injector and provide a signal from which the amount of fuel injectedinto the engine may be determined to provide said feed-back signal.

In another embodiment the or one or more of the injectors through whichpilot diesel fuel is injected into the engine may be provided with meansto detect when and/or the extent to which a valve of the or eachinjector is open and provide a signal from which the amount of fuelinjected into the engine may be determined to provide said feed-backsignal.

In another embodiment differential pilot diesel fuel flow means may beprovided to determine the difference between the amount of pilot dieselfuel supplied to the injector system and the amount of pilot diesel fuelreturned from the injector system and provide a signal from which theamount of fuel injected into the engine is derived to provide saidfeed-back signal.

The control member may be positionable by power operated means such as astepping electric motor, a DC electric motor, a hydraulic or pneumaticram or a diesel controlled actuator.

The means which arrest movement of the control member may comprise amember which mechanically engages the control member or means todisengage the power means.

Because no restraint is placed on the control member for moving towardsits minimum flow position, in the event of the governor which controlsthe position of the control member requiring movement of the controlmember in the direction to reduce the supply of diesel fuel, this cantake place. For example, in the event of failure of the gas supplymechanism in a full flow position, the engine will not overspeed becausethe governor will operate to reduce the amount of diesel supplied and somove the control member to a position at which no diesel oil is suppliedand, therefore, there will be no diesel supplied to initiate ignition ofthe gas so the engine will slow down to its governed speed.

Alternatively, the injection pump may be of the type which provides acontinuous flow of pilot diesel fuel which is fed by a continuouslypressurised supply line to the injector system and the injector systembeing arranged to inject discrete volumes of pilot diesel fuel into theor each cylinder of the engine from said supply line.

In this case the feed-back signal is derived from a differential pilotdiesel fuel flow means which determines the difference between theamount of pilot diesel fuel supplied to the injector system and theamount of pilot diesel fuel returned from the injector system to providea signal from which the amount of fuel injected into the engine may bedetermined to provide said feed-back signal.

The engine may be provided with a fuel control system comprising flowcontrol means adapted to control the relative proportions of each fuelsupplied to the engine, wherein the system includes means to provide anerror signal derived from engine speed and throttle position and meansresponsive to the error signal to supply the diesel and gas fuels tocause the engine speed to tend towards a speed demanded by the throttleposition.

The system may include an engine speed sensor to provide an engine speedsignal responsive to engine speed, a throttle position sensor, toprovide a throttle position signal responsive to the throttle position,the control means being responsive to the engine speed and throttleposition signals to provide a first fuel control signal and a secondfuel control signal to the diesel fuel and gas fuel supply systemsrespectively to cause the engine speed to tend towards said speeddetermined by the throttle position.

Said control means may comprise an electronic signal processing meanshaving a programmed control algorithm in which the difference betweensaid throttle position and engine speed is calculated and used as anerror signal.

The second fuel signal provided by the electronic signal processingmeans may be electronically compared with a pressure signal dependentupon the pressure of the gas fuel supply and a gas supply valve iscontrolled in dependence on a predetermined relationship between saidpressure signal and the gas fuel supply control signal.

The electronic signal processing means may include a look-up tablecontaining information concerning gas flow as a function of a valveopening and gas fuel pressure whereby the look-up table gives therequired valve opening for the required gas flow and the detected gaspressure.

Additional stored information may be used with the error signal todetermine the relative proportions of the diesel and gas fuels, whichmay ensure that the diesel fuel is supplied at a constant orsubstantially constant rate, and the second fuel at a variable rate.This may be at least when the engine speed is above a threshold wherethe position of the control member is arrested.

A sensing means may be provided to sense the operating condition of theengine, which sensing means may provide a signal to the control meanswhich determines the position at which the control means is arrested inaccordance with said stored information.

Where a feed-back signal generating means is provided, the feed-backsignal may also be fed to the control means or an auxiliary controlmeans, which causes the position at which the control member is arrestedto be adjusted in accordance with the required and actual amounts ofdiesel fuel delivered to the engine.

Thus, after initial calibration, it is not necessary to recalibrate theengine. It will be appreciated that as the engine is used, the amount ofdiesel fuel fed to the engine with the control member in its arrestedposition may vary, as seals become worn for example, or as the injectorscarbonise. By providing a feed-back signal as described it can beensured that the amount of fuel delivered for the given condition of theengine is correct. Further, any inaccuracy in the power operated means,where provided, can be accommodated.

The invention is based on our discovery that the increase in the amountof fuel injected with increase in engine speed is due to leakage ofpilot fuel between the piston and a sleeve of the injection pump. Thefaster the engine speed the less time there is for this leakage to occurand thus more pilot fuel is injected into the engine at higher speeds.If, therefore, the rack is fixed in a position to give the amount ofpilot fuel necessary for satisfactory ignition at, for example, lowspeeds, it will inject more pilot fuel at high speeds than is requiredfor ignition and thus the proportion of pilot fuel to gas in the fuel athigh speeds is greater than is necessary.

It has also been found that there is some merit in injecting a largeramount of pilot fuel per piston stroke at low engine speed than isrequired at high engine speed in order to give satisfactory throttleresponse and engine acceleration.

An example of the invention will now be described with reference to theaccompanying drawings wherein:

FIG. 1 is a diagrammatic plan view of a dual fuel diesel engineembodying the invention;

FIG. 2 is a side elevation, to an enlarged scale, of the injection pumpof the engine of FIG. 1;

FIG. 3 is a schematic diagram of one embodiment of a fuel control systemused for the engine of FIG. 1;

FIG. 4 is a diagrammatic cross-section through part of the engine ofFIG. 1;

FIG. 5 is a diagrammatic side elevation of an injector for use in amodification of the engine of FIG. 1;

FIG. 6 is a diagrammatic cross-section through an injector for use inanother modification of the engine of FIG. 1; and

FIG. 7 is a diagrammatic side elevation of an alternative embodiment ofdual fuel diesel engine.

Referring to the drawings, in FIG. 1 a conventional six cylinder dieselengine is indicated at 10 and can comprise, for example, the engine of amotor vehicle such as a lorry. The engine 10 has a conventional fuelinjection pump 11 of the type which pumps discrete volumes or "slugs" ofdiesel fuel to conventional injectors 12 by which the diesel fuel oil isinjected into the cylinders of the engine. Reference may be made to ourpublished specification No. GB-A-2166267 for a more detailed descriptionof such a pump. The diesel oil is supplied to the injection pump 11 by alift pump 13 from a diesel oil tank 15 of the vehicle via a pipeline 16and excess diesel oil from the injectors is returned to the tank 15 viareturn pipe 16a. Air is inducted from atmosphere via an air intake 17and fed to the cylinders of the engine by means of a conventionalmanifold 18. Between the air inlet 17 and the manifold 18 is a duct 19to which gas fuel is fed along a conduit 20 from a gas supply valve 21which is supplied with gas from a storage vessel 22 along a conduit 23via an excess pressure relief valve 24 and a pressure regulator 25.

The gas supply control valve 21 may be controlled by any suitable means,for example, as described below with reference to FIG. 3. This fuelcontrol system includes an engine speed transducer 1' for convertinginformation about engine speed into an electrical signal, a throttleposition transducer 2' for converting information about the position ofthe throttle into an electrical signal, and a gaseous fuel pressuretransducer 3' for converting information about the pressure of thegaseous fuel in conduit 23 into an electrical signal. The engine speedsignal is transmitted along a line 4', and the throttle position signalis transmitted along a line 5' and the pressure signal is transmittedalong line 6' to an input filter 7 of a micro-processor 8' shown indotted lines).

The input filter 7' is a low pass filter and serves to remove anytransient noise present in the various signals and to increase theresolution of the signals. The filtered engine speed signal istransmitted along a line 9' to a proportional, integral and derivativecontrol algorithm designated 10'. The filtered throttle position signalis transmitted along line 11' to the control algorithm 10'. Thedifference between the throttle position and engine speed signals iscalculated within the control algorithm 10' and is used as an errorsignal in the control algorithm.

The control algorithm 10' calculates the required flow rate of fuel andthe relative proportions between the gas fuel required and the dieseloil required. The value of the calculated flow rate is compared with themaximum allowable flow rate for the particular engine speed (which isknown for any particular engine and engine speed) and if the calculatedflow rate is greater than this maximum value, then the value of thecalculated flow rate is replaced with the maximum value.

The control algorithm 10' produces two outputs, one of which isrepresentative of the required diesel fuel flow rate. This signal istransmitted along a line 12' to an electro-mechanical device 13' whichis connected to the diesel injection pump 11 or to an engine speedgovernor which is connected to the injection pump 11.

The engine speed governor may be a conventional governor whichdetermines the amount of fuel which sould be supplied to the cylinderthrough the injection pump 11. This amount of fuel would be the amountof fuel required if the engine was being operated as a diesel onlyengine. When two fuels are being used, the amount of diesel oil requiredby the engine is different from when only one diesel oil is being used.

The electro-mechanical device 13' is adapted to prevent the injectionpump 11 from delivering the quantity of diesel oil which is demanded bythe governor by controlling movement of a control member 11a of the pump(see FIG. 2) in order to control the amount of diesel oil supplied tothe engine. Typically the control algorithm 10' operates such that theeletro-mechanical device 13' prevents the injection pump 11 fromdelivering more than a fixed percentage of the diesel oil which would besupplied at full throttle. An example of a typical percentage isapproximately 10%. The device 13' does not prevent the pump 11 fromdelivering less than this fixed percentage.

The precise percentage of diesel oil injected into each cylinder may befixed throughout the speed range of the engine or it may be variedsomewhat so that the appropriate amount is injected to give satisfactoryignition over the whole speed range of the engine. Because, as explainedhereinbefore, we have discovered that as the engine speed increasesthere is less time for leakage of fuel to occur in the pump and thusmore fuel is injected into the engine as speed increases, if the controlmember 11a is maintained in a constant position, the device 13' may bearranged to cause the control member 11a of the pump 11 to move in thedirection to reduce fuel supply as the engine speed increases, eventhough this can have the effect of maintaing the actual amount of dieselsupplied into the cylinder of the engine constant. The device 13',therefore, adjusts the position of the control member 11a of the pump 11appropriately to give precisely the amount of diesel fuel which it isrequired to be injected which, as explained, can be a constant amountover the whole speed range of the engine or a variable amount if, forany particular engine, it is found that different amounts of pilot fuelare required at different engine speeds.

In all cases the percentage of diesel oil supplied, compared to thatwhich would be supplied at full throttle, is severely restricted and isof the order of 10% in this example, although of course it may be basedon other percentages if desired.

The other output from the control algorithm 10' is transmitted alongline 15' to a look-up table 16'. In addition, the filtered informationconcerning the gaseous fuel pressure is fed along line 17' to thelook-up table 16'. The look-up table 16' contains stored informationnconcerning gas flow as a function of valve opening and gaseous fuelpressure. Consequently the look-up table 16' can calculate the requiredopening of valve 21 given the required gas flow (from line 15') and theexisting gas pressure (from line 17').

The information concerning the valve opening is transmitted along a line18' to a low pass output filter 19', and then along a line 20' to thegas valve 21. The gas valve 21' regulates the supply of gaseous fuelbetween conduit 23 which is connected to the gas storage vessel 22, andconduit 20, which is connected to the duct 19. Reference may be made toour published specification GB-A-2166267 for a more detailed descriptionof the operation of the fuel control system in respect to control of gasflow and the variations therein may be applied mutatis mutandis to thepresent invention. For example, the gas may be supplied via a gasdistributor to a gaseous fuel injector for each engine cylinder.

Referring now particularly to FIG. 2, the diesel injection pump is shownat 11 and the conventional control member or rack is shown at 11a.Carried on a bracket 24a provided on the pump 11 is theelectro-mechanical device 13' which has an output shaft 25a, theposition of which in the present example, is determined by a power meanscomprising a stepping electric motor under the control of the signaltransmitted along the line 12'. The output shaft 25a carries a lever 26which engages the control member 11a to arrest movement of the controlmember 11a in the direction to increase the amount of diesel fuelsupplied by the pump 11, the position of arrest of the control member11a being determined as described hereinbefore. No restraint is placedupon the control member 11a from retracting towards its minimum flowposition.

Thus, in the event of any failure of the gas flow mechanism in the"open" position, the engine will not overspeed. The mechanical governorcontrolling the control member 11a, which is an integral part of thediesel fuel injection pump 11, will operate normally and withdraw thecontrol member 11a to a "no flow" condition and thus no pilot fuel willbe available and the engine will slow to equilibrium at its governedspeed, at which point pilot fuel would again be supplied, without loadon it.

The pump 11 is connected to each of the various six cylinders of theengine 10 by a feed line P. In one feed line P', a feed-back signalgenerating means G is provided which is shown in more detail in FIG. 4.

The generating means G comprises a fitting which is connected in thefeed line P', the fitting having an inlet 30 which is connected to afirst feed line part to the pump 11, and an outlet 31 which is connectedto its respective injector 12 via a second feed line part, a passage 33extending through the fitting between the inlet 30 and the outlet 31.

Branching from passage 33 is a secondary passage 34 which commucateswith a chamber 35 provided by a connector 36 which is received in anopening 37 of the fitting.

Within the chamber 35 is positioned a membrane 38 comprising a disc,which isolates chamber 35 and thus the pressure of the diesel fuel inpassage 33 is communicated to the disc 38. A strain gauge 39 is adheredto the disc 38 and an electrical signal is produced in response to thepressure of the diesel in passage 33. This signal is fed via a signalprocessing means H and a low pass filter 7' to the algorithm 10'.

It will be appreciated that each time the injection pump 11 is operativeto pump fuel along the feed line P' in which the generating means G isprovided, the pressure of the fuel within the feed line P' willincrease.

The strain gauge 39 will produce a signal indicating a sharp rise inpressure as the pump begins to pump, before the injector 12 injects thefuel into the engine, followed by a period of high pressure whilst thefuel is injected, followed by another momentary rise in pressure as theinjector again ceases to inject the fuel. Thereafter the pressure willdecrease as the pump 11 discontinues pumping fuel to feed line P'.

Thus, the time for which the injector is operative to inject the fueli.e. the period whilst the injector 12 is operate, can be determined, aswell as an indication of the magnitude of the pressure. By knowing thesize of passage 33, the amount of fuel delivered to the associatedcylinder, and by averaging, to the engine, can be accurately estimated.Thus the algorithm 10' can not only signal device 13' to arrest movementof the control member or rack 11a, but can adjust its position if it isfound that too much or too little fuel is actually being delivered asdetermined from the feed-back signal generated by means G, compared withthe calculated optimum amount of fuel.

In an alternative arrangement, instead of feeding the feed-back signalto the algorithm 10', the signal may be fed to an auxiliary controlmeans which is operative to adjust the position of arrested member 11awhere the amount of fuel actually delivered is found to be differet tothe required optimum amount of fuel.

In use, on initial setting up of the engine, it is necessry to calibratethe fuel feed system, to ensure that the actual amount of diesel fuelbeing delivered into the engine corresponds with the optimum calculatedvalue as determined by the algorithm 10'. Because of the feed-backsignal generated from feed-back generating means G, it is not necessaryafter this initial calibration to recalibrate the engine. Without thefeed-back signal, it will be appreciated that as the engine is used theamount of diesel fuel fed to the engine with the control member in itsarrested position, may vary as parts of the engine wear and as theinjectors carbonise.

By providing a feed-back signal as described, the arrested position ofthe control member 11a can be continually adjusted to ensure that themaximum amount of fuel permitted for the given condition of the engineis never exceeded. Further, any inaccuracy in the stepper motor of thedevice 13', can be accommodated.

In another embodiment shown in FIG. 5, instead of a strain gauge the orone or more of the injectors 12 through which pilot fuel is injectedinto the engine, may have an accelerometer 40 attached thereto to sensethe opening and/or closing of the injector and provide a signal fromwhich the amount of fuel injected into the engine may be determined andhence provide a feed-back signal.

In another embodiment, shown in FIG. 6, the one or more of the injectors12 through which pilot diesel fuel is injected into the engine isprovided with means 51 to detect when the injector needle valve is openand thus fuel is being injected and so provide a feed-back signal. Themeans 51 may comprise an electrical switch actuated by movement of theneedle 52, the switch 51 being connected by lead 53 to the controlsystem. In a modification the means 51 may not only simply detect whenthe injector needle valve is open but may also detect the extent towhich the needle valve is open and thus provide a more accurate signalof the amount of fuel which is being injected into the engine. In thismodification the means 51 may for example be a potentiometer device.

In another embodiment, shown in FIG. 1, a feed-back signal as describedabove is provided by determining the different between the amount ofpilot diesel fuel being supplied to the injector system and the amountof pilot diesel fuel being returned from the injector system. This isdone by providing flow transducers 60, 61 in the flow line 16 and returnline 16a respectively. The transducers 60, 61 are shown in dotted linein FIG. 1. By comparing the amount of fuel flowing as determined by thetransducers 60, 61 a measure of the amount of fuel injected into theengine is obtained and this provides a feed-back signal which is used,as described hereinbefore, to adjust the position of arrest of movementof the rack 11a in a similar manner to the feed-back signal generated bythe means G described hereinbefore.

In another embodiment, shown in FIG. 7, in which the same reference hasbeen used for corresponding parts as were used in FIG. 1 but with theaddition on 100, the engine 100 is provided with an alternative form ofinjection pump 111 of the type which provides a continuous flow ofpressurised diesel fuel to provide a continuously pressurised line 111aby which pilot fuel is fed to the injectors 112. The injectors 112 areof a different type to that described hereinbefore and instead of beingactuated by the arrival at an injector of a slug diesel fuel, as in thepreviously described embodiments, the injectors are opened at theappropriate time in the engine cycle by a cam shaft and permit injectionof a discrete volume of diesel fuel into the associated cylinder fromthe pressurised line in accordance with the time for which the injectoris maintained open by the cam shaft and the flow characteristics of theinjector.

Such a pressurised line injection system is commonly known as a"Cummins" system.

The dual fuel supply means described hereinbefore in connection withFIG. 1 is used, with appropriate modification, in the presentembodiment. Thus, engine speed and throttle position sensors 101' and102° are provided which provide an input to the microprocessor 108' asis a signal indicative of gaseous fuel pressure difference across thegaseous fuel pressure control valve 121 by a line 106'. In addition,differential flow transducers 160 and 161 provide signals to themicroprocessor 108' indicative of the actual amount of diesel fuelinjected into the engine.

The control algorithm of the microprocessor 108' provides an outputsignal as described in connection with the first embodiment to the gascontrol valve 121' so that the gas supply is controlled as describedpreviously. In addition, the algorithm provides an output signal alongline 112' to suitable means to limit the amount of diesel fuel injectedby the injectors 112 to a pilot amount similar to the amount of pilotdiesel fuel described hereinbefore with reference to the previousembodiments.

The signal provided along line 112' is dependent on the speed sensor101' which provides a first means which provides a first input to thecontrol means comprised by the microprocessor 108' which will cause itto provide a signal along line 112' to cause pilot fuel to be suppliedat a predetermined rate in accordance with the engine speed. However,because as in the case of the first embodiment there are factors wherebythe actual amount of pilot diesel fuel injected into the cylindersdiffers from that intended the flow transducers 160, 161 provide asecond means which provide a second input signal to the control meanscausing modification of the supply of pilot fuel signalled by the firstmeans so that the actually desired amount of pilot diesel fuel is infact injected into the engine. Although, in the above example the firstmeans provides a first input dependent uponn engine speed the firstinput may be dependent on some other engine parameter and similarly thesecond input signal provided by the second means, in the present exampleas a result of the differential flow detected by the transducers 160,161, may be provided as a result of sensing some other engine parameterindicative of the actual amount of diesel fuel injected into the engine.

If desired, instead of providing a stepper motor, a DC motor, or anyother power means may be provided within device 13' to arrest theposition of the control member 11a in the direction to increase thesupply of diesel fuel to the engine. Instead of to six cylinder enginethe invention may be applied to any other type of engine.

If desired, in both the type of injector system described with referenceto FIG. 1 and that described with reference to FIG. 7 a feed-back signalmay be provided by sensing the temperature of the pilot fuel which isindicative of the amount of diesel fuel injected into the engine becausethe amount injected varies with the viscosity of the fuel which varieswith temperature, or by sensing the actual power developed by theengine.

The features disclosed in the foregoing description, or the accompanyingdrawing, expressed in their specific forms or in terms of a means forperforming the disclosed function, or a metal or process for attainingthe disclosed result, or a class or group of substances or compositions,as appropriate, may, separately or any combination of such features, beutilised for realising the invention in diverse forms thereof.

I claim:
 1. A dual fuel compression ignition engine having a gas fuelsupply system for the engine to supply gaseous fuel to the engine at avariable rate whereby the speed of the engine is continuously variable,a diesel fuel supply system for the engine including an injection pumpand an injector system, a control means to control the amount of pilotdiesel fuel injected into the engine, a first means to provide a firstinput to the control means to cause the control means to supply pilotfuel at a variable predetermined rate, said rate being varied inaccordance with at least one operating parameter of the engine andsecond means to provide a second input to said control means from afeed-back signal indicative of the amount of pilot diesel fuel injectedinto the engine and the control means being adapted to adjust the supplyof the pilot fuel to correspond to that signalled by the first input. 2.A dual fuel compression ignition engine according to claim 1 wherein thediesel fuel supply system includes an injection pump having a governor,responsive to engine speed and engine speed demand, to operate a controlmember of the pump which controls the amount of diesel fuel injectedinto the engine by the pump, means to arrest movement of the controlmember of the pump, in the direction to increase the supply of dieselfuel to the engine, so as to supply pilot fuel into the engine at asubstantially constant, rate during dual fuel operation independently ofsaid governor.
 3. An engine according to claim 1 wherein said at leastone operating parameter comprises at least one of the position of thespeed control of the engine and the engine speed.
 4. An engine accordingto claim 1 wherein the injection pump is of the type which pumpsdiscrete volumes of fuel to the injector system.
 5. An engine accordingto claim 1 wherein the injection pump is of the type which provides acontinuous flow of pilot diesel fuel which is fed by a continuouslypressurized supply line to the injector system and the injector systembeing arranged to inject discrete volumes of pilot diesel fuel into theor each cylinder of the engine from said supply line.
 6. An engineaccording to claim 4 wherein the feed-back signal is derived from adifferential pilot diesel fuel flow means which determines thedifference between the amount of pilot diesel fuel supplied to theinjector system and the amount of pilot diesel fuel returned from theinjector system to provide a signal from which the amount of fuelinjected into the engine may be determined to provide said feed-backsignal.
 7. An engine according to claim 5 wherein the feed-back signalis derived from a differential pilot diesel fuel flow means whichdetermines the difference between the amount of pilot diesel fuelsupplied to the injector system and the amount of pilot diesel fuelreturned from the injector system to provide a signal from which theamount of fuel injected into the engine may be determined to providesaid feed-back signal.
 8. An engine according to claim 1 wherein thefeed-back signal is dependent upon the temperature of the pilot fuel. 9.An engine according to claim 5 wherein the feed-back signal is dependentupon the temperature of the pilot fuel.
 10. An engine according to claim1 wherein the feed-back signal is dependent upon the power developed bythe engine.
 11. An engine according to claim 5 wherein the feed-backsignal is dependent upon the power developed by the engine.
 12. Anengine according to claim 1 wherein the engine is provided with a fuelcontrol system comprising flow control means adapted to control therelative proportions of each fuel supplied to the engine, wherein thesystem includes means to provide an error signal derived from enginespeed and throttle position and means responsive to the error signal tosupply the diesel and gas fuel to cause the engine speed to tend towardsa speed demanded by the throttle position and said control meanscomprises an electronic signal processing means having a programmedcontrol algorithm in which the difference between said throttle positionand engine speed is calculated and used as the error signal.
 13. Anengine according to claim 4 wherein the feedback signal is derived froma strain gauge which determines the pressure of the diesel fuel in afeed line from the injection pump, and means to correlate the pressurewith the amount of fuel injected into the engine from the injectionpump.
 14. An engine according to claim 4 wherein the feedback signal isderived from at least one of the injectors, through which fuel isinjected into the engine, provided with an accelerometer to sense atleast one of the opening and closing of the injector and provide asignal from which the amount of fuel injected into the engine may bedetermined to provide said feed-back signal.
 15. An engine according toclaim 4 wherein the feedback signal is derived from at least one of theinjectors, through which pilot diesel fuel is injected into the engine,provided with means to detect at least one of when and the extent towhich a valve of said at least one injector is open and provide a signalfrom which the amount of fuel injected into the engine may be determinedto provide said feed-back signal.